Grain-oriented electrical steel sheets are used as core material in electrical equipments such as transformers and large-sized rotational machines. It is required for the grain-oriented electrical steel sheets having such application uses that they have magnetic characteristics of satisfactory exciting property and low core loss. Among all, core material of low core loss has been required in view of energy saving in recent years.
In the conventional process for producing grain-oriented electrical steel sheets, ingots or slabs obtained by continuous casting have been used as starting material. For obtaining products having excellent magnetic properties from such starting material, there has been employed a process of heating the starting material at high temperature thereby completely solidifying inhibitors such as AlN or MnS, and then finely precipitating them. Accordingly, the hot rolling is an essential step.
On the other hand, in a process of obtaining a thin cast sheet by a rapid quench-solidification process as in a sheet casting process, for example, by using twin rolls, a thin cast sheet is cooled after solidification at a cooling rate of greater than 0.05.degree. C./sec. at least to 600.degree. C., thereby refining the crystal grains, and the thin cast sheet is re-heated in the subsequent step, thereby finely dispersing precipitate as disclosed in Japanese Patent Laid-Open Publication Sho 53-97923 and 54-83620. However, these publications show nothing about the crystallographical texture of the thin cast sheet and an appropriate reduction rate during cold rolling after the casting, which are important factors for the secondary recrystallization of material.
Further, a process for producing grain-oriented electrical steel sheets is also disclosed in Japanese Patent Laid-Open Publication Sho 63-11619 and 63-176427, in which molten metal containing 2.5 to 6.5% by weight of Si is continuously supplied on a cooling body having a cooling surface that is moved and refreshed, for example, a twin roll caster, and solidified by quenching into a thin sheet of 0.7 to 3.5 mm thickness. Then, the thin cast sheet is cold rolled under a reduction rate of not less than 50% and then annealed.
In the prior art described above, the rapid quench-solidification is applied for making the crystal grains finer and the cold rolling under a reduction rate of not less than 50% is applied for finely dispersing precipitate along with the annealing applied subsequently, but they do not at all show that these measures are taken for the crystallographical texture of the thin cast sheet which is an important factor for the secondary recrystallization of material;
Further, Japanese Patent Laid-Open Publication Sho 56-158816 discloses a process for producing grain-oriented electrical steel sheets, including steps of continuously casting molten metal containing less than 4.5% by weight of Si into a thin cast sheet of 3 to 80 mm thickness, and then applying hot rolling under a reduction rate of not less than 50% in a temperature range not lower than 700.degree. C. to obtain a hot-rolled steel sheet of 1.5 to 3.5 mm thickness. In this publication, it is described that no satisfactory secondary recrystallization is formed if the thin cast sheet is not hot rolled due to insufficiency of Goss nuclei and, accordingly, products with satisfactory magnetic properties cannot be obtained.
In the process for producing grain-oriented electrical steel sheets by the rapid quench-solidification process utilizing the inhibitor as described above, it is not clear that this is for the crystallographical texture of the thin cast sheet and appropriate reduction rate in the cold rolling for developing the secondary recrystallization also in a case of saving the hot rolling step, required for obtaining products having satisfactory magnetic properties.
Further, although it has been well-known to reduce the thickness of the products as a means for lowering the core loss in the grain-oriented electrical steel sheets, it has been difficult from an industrial point of view. That is, in a case of reducing the plate thickness by chemical polishing, production yield is remarkably lowered. Further, if the plate thickness is reduced by rolling, there has been a problem that the formation of the secondary recrystallization is difficult.
Japanese Patent Laid-Open Publications Sho 59-126722 and 61-79721 disclose a method of applying two stage cold rolling after hot rolling in order to stably form secondary recrystallization in the grain-oriented electrical steel sheets of a reduced plate thickness. However, the prior art involves a problem that the production cost is increased since two stage cold rolling is necessary. Further, Japanese Patent Laid-Open Publications Sho 61-217526 and 61-238916 disclose a method of applying CBS (contact-bend-stretch) rolling to the hot-rolled material, so that the rolled crystallographical texture is improved and the secondary recrystallization can stably be formed even if the plate thickness is less than 0.18 mm. However, since the prior arts require a special rolling machine, there has also been a problem that the production cost is increased. Further, Japanese Patent Laid-Open Publication Sho 61-238939 discloses a method of forming a thin cast sheet from molten metal by way of a rapid quench-solidification process and then applying cold rolling at least for once under a reduction rate of 55 to 80% as a means capable of stably forming secondary recrystallization even in a case where the plate thickness is less than 0.15 mm. However, in the case of the prior art, there has been a problem that it is difficult to obtain products of high magnetic flux density due to the low reduction ratio in the final cold rolling.